UK customs, how to get information about the package?

Question

Delivery

History
Sent a parcel to England, she left the country, Royal Mail says that it is not registered in their system. Option 2:
1) Stuck somewhere at the terminal
2) Got to customs (customs clearance, etc.)
Who faced? How to find out if the package is detained by customs? What type of letter should be sent to the recipient from customs that the parcel is delayed?
I sell on eBay and make a partial refund if the package is detained by customs (customs is not profitable in the UK for the buyer). But some try to get a refund by receiving the package and dropping left letters from customs .. Due to the fact that the track information is updated on the site only after the package has been delivered.